The invention relates to a method of stress-optical force measurement in which linearly polarised light is introduced into a stress-optical measurement array which is influenced by the force to be measured, so that the polarisation state of the light is changed in a force-dependent manner and is determined in a subsequent evaluation unit. The invention also relates to a measurement device for performing the method comprising a light source for linearly polarised light, a stress-optical measurement array which is arranged between two plates which are exposed to the force to be measured, and a subsequent evaluation unit.
Various force measurement methods and devices are known.
For example, measurement devices for determining a force exerted by weight generally comprise a force sensor as well as a mechanical system consisting of levers, guide elements, springs and the like in order to transfer the force exerted by the weight of, for example an object to be weighed to the force sensor so that the transfer is independent of the position of the object on a weighing apparatus. Therefore, mechanical stability of the measurement device and measurement accuracy can be ensured only with great expenditure.
From DE-OS 29 05 314 there is known a measurement device which has a substantially simpler mechanical construction. Therein, the force exerted by the weight of the object to be weighed is transferred directly to a plurality of force sensors which at the same time stabilise the weighing apparatus or its measurement platform. In a measurement device of this kind it is problematic that the force sensors must have the same sensitivity or that the spread in their, sensitivity, customarily introduced by manufacturing tolerances, must be compensated for in an evaluation unit before the signals from the individual force sensors can be electronically added, thus ensuring independence from the position of the object to be weighed. Considerable expenditure is required for this purpose.
From DE-OS 31 29 847 there are known a method and a measurement device in which linearly polarised light is applied to the stress-optical material. The linearly polarised light is converted into elliptically polarised light when it irradiates a body exhibiting birefringence under the influence of the force to be measured. An analyser then detects a phase shift and hence indirectly the force to be measured. Application of the force to such a device comprising only one force sensor, for example in weighing apparatus, remains problematic. The use of several of such devices, comprising one force sensor each, however, involves great expenditure.
EP O 153 997 discloses a method and a device for stress-optical force measurement, for example for a weighing apparatus. Therein, linearly polarised light is applied to a stress-optical fibre, after which it is evaluated. The optical fibre is arranged in the form of a circle between two parallel plates which are arranged at a distance from one another. The force to be measured acts on one of the plates. Even though in principle a simpler construction without complex calibration is thus ensured, problems are encountered as regards the mechanical construction and the coupling-in and coupling-out of the light because of the very small thicknesses of the sensitive optical fibre.